The synthesis and gas permeation properties of addition-type poly(3,3-bis(trimethylsilyl)tricyclononene-7) (PTCNSi2g) are first reported. High molecular weight PTCNSi2g was obtained via addition polymerization of 3,3-bis(trimethylsilyl)tricylononene-7 on a Pd-containing catalyst. It possessed a BET surface area as high as 790 m(2)/g. This new polymer is distinguished by extra high gas permeability and solubility controlled permeation of hydrocarbons. Positron annihilation lifetime spectroscopy revealed extremely large size of free volume elements (8.3 angstrom). PTCNSi2g is a promising membrane material for separation of natural gas.